This invention relates to a television signal amplitude detection circuit for detecting the amplitude of an input television signal.
With recent advancement of image processing technique and semiconductor technique, image processing systems making use of image processing technique have been developed in various fields. In these image processing systems, their characteristics and performance are liable to be impaired if the amplitude of the input television signal becomes excessive or insufficient with respect to a prescribed television signal amplitude range. For this reason, determination as to whether the input television signal amplitude is within the prescribed amplitude range is important in order to maintain satisfactory characteristics and performance of the image processing system.
The television signal amplitude is prescribed such that a ratio of the video signal amplitude to the sync signal amplitude is 0.7:0.3. However, the video signal amplitude varies between 0 and 0.7 V, so that the detection as to whether the television signal amplitude is within the prescribed amplitude range is usually done with respect to the horizontal sync signal amplitude. However, in the color television signal the horizontal synchronizing signal interval (including a front porch, a horizontal sync signal period and a back porch) contains the color burst. Therefore, where the sync signal amplitude detection is made within the horizontal synchronizing signal interval, only a very short period is available for the sync signal amplitude detection. This means that a sync separator capable of accurately detecting the horizontal sync signal position in the television signal is necessary, thus complicating the circuit construction.
Such a prior art signal detection circuit is seen in U.S. Pat. No. 3,706,847 wherein the peak of a horizontal sync signal fed from a terminal (reference numeral 1 in FIG. 1) is detected by a peak detector (9), and the porch level is detected by a porch detector (5), whereby a medium value of outputs of these two detectors is detected by a circuit (19).
Further, in the case of a simplified color television camera, color burst sometimes fails to be inserted in a regular position. In such a case, erroneous detection of the sync signal amplitude would result even by using a sync separator which can accurately detect the horizontal sync signal position.